Method of Manufacturing Zirconium Alloy Tubular Products

ABSTRACT

Method of manufacturing zirconium alloy tubular products containing (wt. %): niobium—0.9-1.7; iron—0.04-0.10; oxygen—0.03-0.10; silicon—less than 0.02, carbon—less than 0.02, and zirconium—as the base of the alloy. This includes an ingot melting by multiple vacuum arc remelting, mechanical processing of the ingot, heating, hot working of the ingot, subsequent mechanical processing for the production of tubular billets, heat treatment of the tubular billets, application of a protective coating and heating to a hot pressing temperature, hot pressing, removal of the protective coating, multi-stage cold radial forging, vacuum thermal treatment, multiple cold rolling runs with a total deformation degree of 50-80-% per run and a tubular coefficient of Q=1.0-2.7 with intermediate vacuum thermal treatment after each cold rolling operation, and final vacuum thermal treatment of the resulting tubular products carried out at the final size with subsequent final finishing operations.

TECHNICAL FIELD OF THE INVENTION

The invention is referred to the nuclear technical field, particularlyto the method of manufacturing zirconium alloy tubular products used ascladding and pressure tubes in water-cooled nuclear reactors, includingthe reactors of WWER type.

PRIOR ART

Zirconium alloys are used as the materials for structural components ofnuclear power reactors due to their unique properties: low thermalneutrons absorption section, corrosion resistance in high-temperaturewater and the water steam medium, oxidation and hydrogen absorptionresistance, low irradiation growth and other physical and mechanicalproperties. The properties of the tubular products depend on thechemical composition and each process operation from the ingot meltingto the final finishing operations.

The patent “Method of Manufacturing Zirconium Alloy Tubular Products(Variants)” (RU 2123065C1 published on Mar. 12, 1997, cl. C22F/1/18),including production of the primary blank, production of the tubularbillet, cold rolling of the tubular billet with intermediate and finalannealing for binary zirconium alloy is already known.

The drawbacks of this method reside in the fact that no protectivecoating is applied on the billet prior to hot extrusion resulting inoxidation of the metal in the course of manufacturing process andreduction of the processability of tubular products, and no finalfinishing operations enabling to remove residual process contaminationsfrom the surface of tubular products and to reduce the surface roughnessare provided thus decreasing corrosion resistance of the products.

The patent “Method of Manufacturing Zirconium Alloy Products” RU2110600C1 (published on May 10, 1998, cl. C22F/1/18) includingmanufacturing of the primary blank from the ingot by thermal forming,subsequent production of the intermediate billet by thermal forming,quenching and tempering of cut-to-length sections, thermal forming andtempering prior to cold rolling and performance of cold rolling isalready known.

The drawbacks of this method reside in the fact that no protectivecoating is applied on the ingot prior to hot extrusion resulting inoxidation of the metal in the course of extrusion process and reductionof the processability of tubular products, and no final finishingoperations enabling to remove residual process contaminations from thesurface of the tubular products and reduce the surface roughness areprovided thus decreasing corrosion resistance of the products.

The patent “Zirconium Alloy with the Improved Corrosion Resistance forFuel Element Claddings and Their Manufacturing Method” US 2016/0307651A1(published on Oct. 20, 2016, cl. G21C 3/07, B22D 21/00, B22D 7/00, C22C16/00, C22F 1/18) is the closest to the claimed method. The methodspecifies the composition of corrosion-resistant zirconium alloy and themethod for manufacturing of fuel element claddings made of this alloy,including the ingot melting, the ingot coating with the protective steelshell, thermal treatment of the ingot together with the shell prior tohot rolling, the hot rolling, removal of the protective steel coating,thermal treatment of hot-rolled tubular billets, three runs of coldrolling, intermediate thermal treatment after each rolling and finalthermal treatment.

The main drawback of the method is insufficient processing of thematerial at the cold rolling stage with the total rolling draft of up to60% per a run resulting in incomplete elimination of non-uniformhot-rolled structure. The drawbacks of the method also include:application of the carbon-containing steel shell interacting withzirconium alloy at the hot rolling temperature with potential generationof carbides. Moreover, the material recrystallization degree is one ofthe main factors defining processability and deformation resistancecharacteristics (resistance to thermal, radiation and thermal creep aswell as to irradiation growth) of zirconium alloys. Low temperatures ofthe intermediate annealing (570° C. to 590° C. for the 1-st run, 560° C.to 580° C. for the 2-nd run, 560° C. to 580° C. for the 3-rd run) forthe selected deformation manufacturing scheme (30-40% of deformation atthe first and the third stages, and 50-60%—at the second stage of colddeformation) are insufficient for relaxation of residual stresses andcompletion of recrystallization processes that affects not only thematerial processability but also its deformation resistancecharacteristics, particularly under the impact of radiation. Use of thethree-level long-term final annealing (1-st level—460° C. to 470° C.,2-nd level—510° C. to 520° C., 3-rd level—580° C. to 590° C.) enables toachieve the increased material strength level; in this case thedeformation resistance characteristics are deteriorated primarily due toincomplete recrystallization process. The process flow diagram does notprovide for any final finishing operations enabling to remove residualprocess contaminations from the surface of tubular products and reducethe surface roughness thus decreasing corrosion resistance of theproducts.

SUMMARY OF THE INVENTION

The object of this invention is to develop the method of manufacturingzirconium alloy tubular products of various diameters that can be usedas cladding tubes in water-cooled nuclear reactors.

The technical result is improved processability of the material at allstages of hot and cold pressure shaping applied in the course of tubularproduct manufacturing as well as high corrosion resistance of thetubular products with stable characteristics of mechanical propertiesand deformation resistance.

The technical result is achieved with respect to the method ofmanufacturing tubular products of zirconium alloy containing (% wt.):niobium—0.9-1.7; iron—0.04-0.10; oxygen—0.03-0.10; silicon—less than0.02, carbon—less than 0.02, zirconium—all the rest, including the ingotmelting by multiple vacuum arc remelting, mechanical processing of theingot, heating, hot working of the ingot, subsequent mechanicalprocessing for production of tubular billets, heat treatment of tubularbillets, application of the protective coating and heating to the hotpressing temperature, hot pressing, removal of the protective coating,multi-stage cold radial forging, vacuum thermal treatment, multiple coldrolling with the total deformation degree of 50-80% per a run and thetubular coefficient of Q=1.0-2.7 with intermediate vacuum thermaltreatment after each cold rolling operation, and the final vacuumthermal treatment of the resulting tubular products is carried out atthe final size with subsequent final finishing operations.

Hot working is carried out by multi-stage forging or screw rolling inthe temperature range from 980° C. to 700° C. with a total deformationdegree of 67-83% and with intermediate heating at the temperature from850° C. to 800° C.

Tubular billets are produced by drilling and subsequent boring of theaxial center hole in the ingot divided into definite cut length afterthe hot working.

Thermal treatment of the tubular billets is carried out at thetemperature from 730° C. to 780° C.

Hot pressing of the tubular billet is carried out at the reheattemperature from 750° C. to 650° C. and the elongation ratio ofμ=8.9-12.9.

Multi-stage cold radial forging of tubular billets is carried out withthe rolling draft of 33%.

Vacuum thermal treatment of the tubular billets in the intervals betweencold rolling and final vacuum heat treatment is carried out at thetemperature of 565-630° C.

Vacuum thermal treatment is carried out at the residual pressure of1·10⁻⁴-1·10⁻⁵ mm Hg in the furnace.

Chemical and mechanical processing of the surfaces is carried out at thefinal size of the tubular products.

The selected proportion of alloying components in the zirconium alloyprovides for the processing properties, corrosion resistance, stablecharacteristics of mechanical properties and deformation resistance ofthe tubular products.

The advantage of the tubular product manufacturing in accordance withthe claimed method resides in the fact that hot working of the ingot(forging or screw rolling) ensures uniform processing of the caststructure along the ingot length and cross-section, and application ofthe copper protective coating provides for protection against gas pickupand prevents diffusion interaction between the coating and the billet.Cold rolling with intermediate thermal treatment provides forhomogeneous recrystallized structure of the tubular products with highmechanical properties and also the required anisotropy of properties inthe transverse and longitudinal direction. Final finishing operationsprovide for the roughness Ra of less than 0.8 μm on the outer and innersurface thus increasing stability of the corrosion properties. The innersurface roughness enables to improve the processes of fuel pelletsloading into the tubular products.

EMBODIMENT OF THE INVENTION

The method is embodied in the following way:

Example 1

In accordance with the claimed technical solution the technology ofmanufacturing zirconium tubular products includes the followingoperations. Melting of the alloy ingot consisting of: niobium—0.97-1.03%wt., iron—0.080-0.010% wt., oxygen—0.040-0.045% wt.,silicon—0.003-0.004% wt., carbon—0.0044-0.0046% wt., zirconium—all therest. The initial alloying components are mixed with electrolyticzirconium powder, and then consumable electrodes are formed and meltedby two-stage vacuum arc remelting. The ingot is processed mechanically.The ingot is heated to the temperature from 980° C. to 930° C. in theelectric resistance-type furnace. Multi-stage forging or screw rollingof the ingot after heating is carried out within the temperature rangefrom 980° C. to 700° C. with intermediate heat-up in the electricresistance-type furnace within the temperature range from 850° C. to800° C. The total deformation Σε in the course of hot working of theingot lies in the range from 67 to 83%. The ingot is divided into thedefinite cut length in the size of Ø249×43 mm or Ø199'36.5 mm andprocessed mechanically, and the tubular billets are obtained by drillingand subsequent boring of an axial central hole in them. Thermaltreatment of the tubular billets at the temperatures from 730° C. to780° C. The roughness of the surface of the tubular billets is no morethan R_(a)=2.5 μm. Then copper coating is applied on the tubular billetsin order to protect them against gas pickup in the course of subsequentheating and hot pressing. Heating of the tubular billets for hotpressing is carried out in a combined method, first in an inductionfurnace, and then in an electric resistance furnace to equalize thetemperature along the height and cross-section of the tubular billet.The heating temperature of the tubular billet prior to pressing iswithin the range from 650° C. to 750° C. Pressing is carried out withthe elongation ratio of μ within the range from 11.4 to 12.9. Further,the copper coating is removed and preliminary operations for multiplecold rolling are carried out. To reduce metal losses into chips duringmachining of a tubular billet, multistage radial forging is carried outon an SKK radial forging machine with deformation (ε=33% per pass).Next, the tubular billets are sent for vacuum thermal treatment (T=565°C.). The tubular billets are rolled on cold-rolling mills of the HPT,KPW types in three passes with a total deformation Σε per pass from 60to 80%, while the tubular coefficient Q lies in the range of 1.0-2.7.Intermediate and finishing thermal treatments are carried out within thetemperature range from 590° C. to 630° C. in vacuum with a residualpressure in the furnace not higher than 1·10⁻⁴-1·10⁻⁵ mm Hg. After thefinal vacuum thermal treatment of the tubular products at thetemperature from 590° C. to 630° C., final finishing operations arecarried out: package or jet etching, abrasive processing of the innersurface, grinding and polishing of the outer surface are performed.

Zirconium alloy tubular products manufactured in accordance with theclaimed technical solution are characterized with the followingproperties (Table 1, Example 1).

Example 2

In accordance with the claimed technical solution the technology ofmanufacturing zirconium tubular products includes the followingoperations. Melting of the alloy ingot consisting of: niobium—0.99-1.08%wt., iron—0.051-0.057% wt., oxygen—0.075-0.080 wt., silicon—0.003-0.004%wt., carbon—0.0032-0.0036% wt., zirconium—all the rest. The initialalloying components are mixed with zirconium magnesiothermal sponge, andthen consumable electrodes are formed and melted by two-stage vacuum arcremelting. The ingot is processed mechanically. The ingot is heated tothe temperature from 930° C. to 980° C. in the electric resistance-typefurnace. Multi-stage forging of the ingot after heating is carried outwithin the temperature range from 980° C. to 700° C. with intermediateheat-up in the electric resistance-type furnace within the temperaturerange from 800° C. to 850° C. The total deformation Σε in the course ofhot working of the ingot is 67%. The ingot is divided into the definitecut length in the size of Ø249×49 mm and processed mechanically, and thetubular billets are obtained by drilling and subsequent boring of anaxial central hole in them. Thermal treatment of the tubular billets atthe temperatures from 730° C. to 780° C. The roughness of the surface ofthe billets is no more than R_(a)=2.5 μm. Then copper coating is appliedon the tubular billets in order to protect them against gas pickup inthe course of subsequent heating and hot pressing. Heating of thetubular billets for hot pressing is carried out in an induction furnace,and then in an electric resistance furnace to equalize the temperaturealong the height and cross-section of the billet. The heatingtemperature of the tubular billets prior to pressing is within the rangefrom 650° C. to 670° C. Pressing is carried out with the elongationratio μ equal to 8.9. Further, the copper coating is removed. In orderto reduce metal losses into chips during machining of a tubular billet,multistage radial forging is carried out on an SKK radial forgingmachine with deformation (ε=33% per pass). Next, the tubular billets aresent for vacuum thermal treatment (T=565° C.). The tubular billets arerolled on cold-rolling mills of the HPT, KPW types in four passes with atotal deformation Σε per pass from 50 to 78%, while the tubularcoefficient Q lies in the range of 1.0-2.3. Intermediate thermaltreatments are carried out in the temperature range from 570° C. to 600°C. in vacuum with a residual pressure in the furnace not higher than1·10⁻⁴-1·10⁻⁵ mm Hg. After the final vacuum thermal treatment of thetubular products at the temperature from 590° C. to 595° C., finalfinishing operations are carried out: package or jet etching, abrasiveprocessing of the inner surface, grinding and polishing of the outersurface are performed.

INDUSTRIAL APPLICABILITY

Zirconium alloy tubular products manufactured in accordance with theclaimed technical solution are characterized with the followingproperties (Table 1, Example 2).

Thus, the presented tube manufacturing method enables to produce tubularproducts with high corrosion resistance, stable characteristics ofmechanical properties and deformation resistance.

TABLE 1 Properties of the tubes manufactured of the Zr—Nb system alloyin accordance with the claimed technical solution Number of remeltings/Chemical weight of composition the final Tube Mechanical properties ofthe alloy, remelting dimensions, σ_(b) ^(⊥), σ_(0.2) ^(⊥), δ^(⊥), σ_(b)^(//), σ_(0.2) ^(//), δ^(//), No. % (wt.) ingot, tons mm MPa MPa % MPaMPa % T_(test). = 20° C. 1 niobium - 2 vacuum arc Ø13.58 × 11.70 440-450360-390 32-38 — — — 0.99-1.08; remeltings/3.5 Ø13.00 × 11.00 iron -Ø9.10 × 7.73 0.051-0.057; Ø10.30 × 8.80  oxygen - Ø9.10 × 7.930.075-0.080; Ø8.90 × 7.73 silicon - Ø9.50 × 8.33 0.003-0.004; carbon -0.0032-0.0036; Zr - all the rest 2 niobium - 2 vacuum arc Ø13.58 × 11.70440-450 360-390 32-38 — — — 0.99-1.08; remeltings/3.5 Ø13.00 × 11.00iron - Ø9.10 × 7.73 0.051-0.057; Ø10.30 × 8.80  oxygen - Ø9.10 × 7.930.075-0.080; Ø8.90 × 7.73 silicon - Ø9.50 × 8.33 0.003-0.004; carbon -0.0032-0.0036; Zr - all the rest Corrosion 400° C. τ = Mechanicalproperties 72 hours σ_(b) ^(⊥), σ_(0.2) ^(⊥), δ^(⊥), σ_(b) ^(//),σ_(0.2) ^(//), δ^(//), Weight No. MPa MPa % MPa MPa % gain, RoughnessT_(test). = 20° C. T_(test) = 380° C. mg/dm² Ra, μm 1 190-210 160-19038-48 190-220 108-130 58-63 10-14 Outer surf. < 0.4 Inner surf. < 0.8 2190-210 160-190 38-48 190-220 108-130 58-63 10-14 Outer surf. < 0.4Inner surf. < 0.8

What is claimed is:
 1. The method of manufacturing zirconium alloytubular products containing (% wt.): niobium—0.9-1.7; iron—0.04-0.10;oxygen—0.03-0.10; silicon—less than 0.02, carbon—less than 0.02,zirconium—all the rest, including the ingot melting by multiple vacuumarc remelting, mechanical processing of the ingot, heating, hot workingof the ingot, subsequent mechanical processing for the production of thetubular billets, heat treatment of tubular billets, application of theprotective coating and heating to the hot pressing temperature, hotpressing, removal of the protective coating, multi-stage cold radialforging, vacuum thermal treatment, multiple cold rolling with the totaldeformation degree of 50-80% per a run and the tubular coefficient ofQ=1.0-2.7 with intermediate vacuum thermal treatment after each coldrolling operation, and the final vacuum thermal treatment of theresulting tubular products is carried out at the final size withsubsequent final finishing operations.
 2. The method as per claim 1featuring the hot working is carried out by multi-stage forging or screwrolling in the temperature range from 980° C. to 700° C. with a totaldeformation degree of 67-83% and with intermediate heating at thetemperature from 850° C. to 800° C.
 3. The method as per claim 1featuring the tubular billets are produced by drilling and subsequentboring of the axial center hole in the ingot divided into definite cutlengths after the hot working processing.
 4. The method as per claim 1featuring the thermal treatment of the tubular billets is carried out atthe temperature from 730° C. to 7850° C.
 5. The method as per claim 1featuring the hot pressing of the tubular billet is carried out at theheating temperature from 750° C. to 650° C. and the elongation ratio ofμ=8.9-12.9.
 6. The method as per claim 1 featuring the multi-stage coldradial forging of tubular billets is carried out with a rolling draft of33%.
 7. The method as per claim 1 featuring the vacuum thermal treatmentof the tubular billets in the intervals between cold rolling and thefinal vacuum thermal treatment is carried out at the temperature of565-630° C.
 8. The method as per claim 7 featuring the vacuum thermaltreatment is carried out at the residual pressure of 1·10⁻⁴-1·10⁻⁵ mm Hgin the furnace.
 9. The method as per claim 1 featuring the chemical andmechanical treatment of the surfaces is carried out at the final size ofthe tubular products.